For more than about two or three decades, the art for producing a metallic sintered body from powdered metallic materials has been well known in the field of metallurgy.
This art is generally referred to as injection powder metallurgy or metal injection molding (MIM).
The powdered metallic materials are mixed with a binder and then formed into a desired shape by extrusion molding or injection molding, and/or compression molding, to form what is commonly referred to as a green body.
The green body is heated by exposing it to a high temperatures atmosphere to partially remove or eliminate the binder by oxidation or thermal decomposition of the binder, the resultant product being referred to as a brown body.
The brown body is then sintered to provide a fusion of the powdered metallic materials and to completely remove or eliminate the binder, by exposing it to a very high temperature atmosphere, for example, 600.degree. C. to 1,000.degree. C., thereby producing the desired shaped metallic product with desired surface appearance, strength, etc., this being called a silver body, that is, the desired product.
In the production of the green body in the manner described hereinabove, there have heretofore been used thermoplastics such as polyethylene, polypropylene, polystyrene, polyamide, and cellulose and derivatives thereof; thermosetting plastics such as epoxy resins, phenolic resins, polyimide resins, natural waxes such as animal wax, China wax, wool wax, vegetable wax, carnauba wax, Japan wax, etc., synthesis waxes such as montan wax derivatives, paraffin wax derivatives, microcrystalline wax derivatives, higher monohydric alcohols such as cetyl alcohol, higher fatty acids such as capric acid, and glycerides such as tripalmitin, hydrocarbon waxes such as low molecular weight polyethylene, etc., and a mixture thereof as a binder for a powdered metallic material.
Also, an oxygen-containing wax type binder is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 20775/1983 (U.S. Pat. No. 4,649,003).
Also, ethylene-vinyl acetate copolymer, low density polyethylene, methacrylate copolymer, phthalic ester, etc., and a mixture thereof are disclosed in Japanese Unexamined Patent Publication (Kokai) No. 229403/1984.
However, several facts indicate the need for improvements in such sintering compositions, particularly with respect to the binder.
A first fact is that a binder as described hereinabove, for example, thermoplastic or thermosetting plastics, have disadvantages such as that a long time is required for the removal or elimination of them.
A second fact is that a binder cannot be removed or eliminated perfectly in the removal or elimination process therefor.
A third fact is that such waxes typically have disadvantages such as low mechanical strength in a green body.
A fourth fact is that a binder composition is composed of complicated components.